1. Field of the Invention
The present invention relates to a non-woven fabric comprising a fiber part and a matrix binding the same fibers, characterized by a melange-type visual aspect and by high durability; a procedure for the preparation of such non-woven fabric, as well as means suited to that purpose.
The definition “melange aspect” refers to a particular visual effect characterized by an alternation of color and/or intensity obtained by the simultaneous use of one or more of the possibilities listed below:
1. Intimate mixture of differently dyed fibers
2. The use of a suitable “needle-dyeing” technology that can insert color into the interior of the non-woven fabric
3. Employment of the print technology considered most suitable for achieving the desired effect (ink-jet, roller, screen, etching, etc.)
The definition “high durability” refers to the high resistance of the material to abrasive agents and to its degradation over time due mainly to absorption of ultraviolet radiation and oxidizing agents.
The high-durability properties are obtained by the use of a polyurethane matrix (binding the fiber elements) characterized not only by its high durability, but also by its ability to be processed at temperatures above those which can be tolerated by polyurethanes usually used for similar purposes.
2. Description of Related Art
Micro-fiber-type non-woven fabrics are known that show a structural organization comprising a high-density fiber surface and a matrix able to bind such micro-fiber structure. The micro-fibers making up these materials are polyester and/or polyamide based, the binder matrix is polyurethane-type (where such term means all the polyurethane-polyurea polymers generally used for this purpose). Products of this type, that have appearance and texture similar to that of natural suede leather, are described for example in Italian Patent Nos. 823055, 839921, 858373, 873669, 905222, 921871 and in U.S. Pat. Nos. 3,531,368 and 3,889,292, all in the name of the present Applicant: such patents are cited here to constitute an integral base for this description for every reference of interest.
Non-woven micro-fiber fabrics of the type described are now produced according to a procedure that can be outlined thus:
1. spinning of a bi-element fiber of the “islands-in-the-sea” type, in which the “island” element comprises polyester or polyamide and the “sea” element comprises a polymer which is immiscible in the island element and can be dissolved in suitable organic or inorganic solvents.
2. constructing a felt by means of a process of mechanical needle-punching which can interlace between the micro-fibers obtained at point 1.
3. Impregnating the felt with a binder which holds the “islands” during the “sea” elimination phase. Such a binder is typically an aqueous solution of polyvinyl alcohol (PVA).
4. Dissolving the “sea” element in a suitable organic (generally trichloroethylene) or inorganic (aqueous acidic or basic solution) solvent or simply warm water.
5. Impregnating the micro-fiber element in a solution, emulsion or aqueous dispersion of PU in organic solvents (DMF).
6. Eliminating binder used in point 3 (if binder is not PU).
7. Dividing the bi-element (“island” element+PU) sheet into two equal parts by means of a cut across the thickness parallel to the surface.
8. Buffing the surfaces of the product by means of suitable treatment with abrasive papers, to confer the characteristic suede appearance on the structure.
9. Final dyeing of the product.
The methods used to dye polyester-based non-woven fabric referred to above, provide for dyeing the micro-fiber element by immersing the material in baths containing “dispersed” type coloring agents. It should be emphasized that the use of dispersed coloring alone does not involve dyeing any of the polyurethane matrix, which thus retains its original white or gray color, in as much as this material is not dyeable in a permanent way using this class of coloring. The dyeing process is then followed by a stripping phase, carried out by means of sodium hydrosulphite in NaOH, to remove the excess of coloring present in the material and thus improve the color-fastness characteristics when washed, abraded or exposed to light. The presence of spotting due to the undyed binder matrix sometimes necessitates a second dyeing process capable of dyeing even the polyurethane phase. Such requirement, generally associated with particularly lively and bright colorations, is satisfied by recourse to a second dye bath, downstream of the standard bath described above, in which so-called “pre-metalized” colorants are used capable of dyeing the polyurethane base and thus avoiding the problems related to coloration differences between the micro-fiber and the binder matrix. Such processes enable production of a product surface with a characteristic suede appearance and uniform dye in its micro-fiber element.
Non-woven fabrics with melange appearance are already known, that are produced by using polyurethane matrices that do not give high durability (resistance over time to oxidating agents, U.V. radiation, and hydrolysis of the polymer chains) together with workability at high temperatures (>170° C.) and which necessarily require the sacrifice of at least one of the other desired characteristics in order to confer such porosity characteristics on the end product as to give it a high level of softness.